Step-by-step nonlinear voltage divider for digital photometer



A ril 29, 1969 J. HEKRDLE 3,4

STEP-BY-STEP NONLINEAR VOLTAGE DIVIDER FOR DIGITAL PHOTOMETER Filed Jan.21, 1965 I Sheet of 2 4 41 9 MF M0 GALVA/VUME'TEI? L {I WWWv FIG. 1

INVENTOR.

M m/w A ril 29, 1969 J. HEKRDLE 3,441,835

STEP-BY-STEP NONLINEAR VOLTAGE DIVIDER FOR DIGITAL PHOTOMETER Filed Jan.21, 1965 Sheet of 2 6 P007 02 4 0% 00 am 000 pm I m PI 1 I I l FIG. 3

INVENTOR.

Tan fie/(rd/e BY WM W Unted States Patent US. Cl. 323,-80 2 ClaimsABSTRACT OF THE. DISCLOSURE A step-by-step nonlinear voltage divider fora digital photometer has individual steps of different values whichmaybe further subdivided. Each step comprises a pair of resistors connectedin parallel. The resistance value of one of the resistors is the same inall the steps; An additional voltage divider has an overall-resistancevalue which is: equal to the value of the oneofthe resistors: Aswitching arrangement is connected between the resistors and theadditional voltage divider for disconnecting a selected one of the oneof the resistors from the corresponding other of the resistors andconnecting the additional voltage divider.

The invention relates to a digital photometer, for ex.- ample for filmdosimetry, using two photoelectric cells.

Photometrical methodsare nowused in many industrial branches forchecking and controlling production, development work and research.

Photometers have lately also been used in nuclear physics for measuringionization radiation by the method ofphotographic emulsion. This methodhas also been sufliciently worked out and has been largely used inchecking the safety of work for personal control of the em-, ployees.Blackening of the control films is a measure for ascertaining theradiation doses which the individual employees have received. Themagnitude of the blackening is defined as-the logarithm to the base ofthe ratio-between the intensity'of'the light incident on the film andthe intensity of the light which haspassed through the film. Onemeasuresintegral blackeningon the film surface. The dispersed light. of.a standard incandescent lamp is used for illumination. Hence where S isthe blackening, I is the intensity of the incident light, I is theintensity of the light which has passed through the film.

Sometimes the total film blackening is measured, and sometimes onesubtracts thefundamental blackening of the non-illuminateddevelopedemulsion including the base. Then, if 8,, is the blackening due toionization radiation, S is:the blackening of the-non-illuminated filmand I is the intensity of the light which has passed throughthe-non-illustrated film, one obtains o rgr Thus, the value of theblackening 8,, caused by radiation only is ascertained by merelysubtracting the. fundamental blackeningS from the total blackening S.;

In addition to the method of direct visual comparison which isinfrequently used, methods which primarily utilize equipment withphotoelectric elements are now frequently utilized. The photoelectricelements are of the semiconductor type, or photoelectric cells of thevacuum type' or S=log g l-log %=Sz+Sd of the gas filled type. Thecurrent flowing through the photoelectric cell is determined by theprinciple that the current is linearly proportional to the intensity ofillumination,

where I is the current flowing through the photoelectric cell, It is theconstant, I is the intensity of illumination.

Several types of photometers have been designed using.

this principle and intended for. various purposes, such as for spectralanalysis, for which perfect types of photometers have been constructed.But because of their special application, these instruments are notsuitable for measuring dosimetrical films. They usually measure in avery small area or slot, and the method of reading tires'the eyes inmass measurements and automatic equipment. The registration of' themeasurement for purposes of checking and statistics is thus difficult.

Existing hotometers use either a single photoelectricv cell, or twophotoelectric cells. Photometers of'the-first type are sensitive tovariations in the supply-voltage, more particularly'to variations inthevoltage of the illumination source. This -is a drawback when carryingout measurements extending over a longer'period of time.

Photometersof the secondztype, that is those using two.

photoelectric cells and the bridge principle, are mechanically verycomplicated and expensive. Reading is accomplished on a sensitive mirrorgalvanometer and is very tiresome to the eye, particularly if manymeasurements have to be carried out. In other types, the lightfiuxes arebalanced to zero of the galvanometer by means of mechanical shutters ordiaphragms' or a tone wedge. The construction of such a device is costlyand the measurement tedious, and this is again a drawback if manymeasurements have to be carried out.

It is a general object of the invention to eliminate the aforementioneddrawbacks. The photometer in accordance with the invention uses twophotoelectric cells. It is therefore independent of variations in thevoltage of the illumination lamp. One of the photoelectriccells-operates as a reference element. Its current is amplified by a DO.tube amplifier with feedback. Partof the output voltage is used forcompensation of the second photoelectric cell which operates as ameasuring element.

The fundamental feature of the invention resides in the construction ofan output voltage divider of the amplifier for obtaining thecompensation voltage for the second photoelectric cell.Thisoutputvoltage divider comprises two potentiometers. The first of thesaid potentiometers is connected across the entire output voltage of theamplifier of the first photoelectric cell. The second potentiometer isconnected between the slide contact and the zero end of thefirst'potentiometer. The secondpotentiometer serves for obtaining thevoltagefor compensation of the current of the second photoelectric cell.Its position directly indicates the values S. The second potentiometeris of the step-by-step type, but because it has a linear characteristic,a special-circuit arrangement is used which is also part of theinvention. Each section of the step-by- I step potentiometer comprisestwo parallel resistors. One of these resistors has a constant value forall sections; the second resistor adjusts the total resistance of thesections to the proper value: The first (constant) resistors arealternately disconnected and another voltage divider is connectedinstead of them. The voltage divider comprises ten resistors each ofwhich has a value equal to one tenth part of the value of thedisconnected resistor. Therefore, the change does not affect the totalvalue of the given section. In this manner, even with anonlinear'course, steps are provided in two rows (in the caseconsidered, tenth parts and. hundredth parts of"S-). The error producedby the fact that the various steps of the-nonlinear course or behaviourare further divided into a linear ratio, is small and negligible.

The invention will be best understood from the following specificationto be read in conjunction with the accompanying drawings, in which:

FIG. 1 explains the fundamental principle of the invention;

FIG. 2 illustrates an actual embodiment of the invention; and

FIG. 3 shows a further detail of the invention.

Referring now more particularly to FIG. 1, it should be understood thatthe light of the lamp L falls upon the photoelectric cell F1 and throughthe measured film MF upon the photoelectric cell F2. In accordance withthe Stolet law the current flowing through the photoelectric cells isproportional to the intensity of the incident light, that is The currentof the comparison photoelectric cell is amplified by the DC tubeamplifier Z. By means of the resistor R 100% negative feedback isintroduced into this amplifier so that the voltage amplification A ofthe amplifier is approximately 1, and the error is of the order 0.1%. Bymeans of the resistive voltage divider a portion U is taken of theoutput voltage U,, to compensate the current i flowing through thephotoelectric cell F2 so that the voltage across the input of thegalvanometer H is zero. The output voltage U of the amplifier Z is Uv U==I R that t., The necessary compensation voltage is [& U -tR that Is zzBlackening of the film is then given by the relation o o v t; g L S-logI log -log -log -g-l-log Uk If the logarithms are to the base inEquation 4, the characteristic S is provided.

In the actual embodiment illustrated in FIG. 2, conditions are somewhatmore complicated. For structural reasons it cannot be reliablyguaranteed that the intensity of the light I incident on the film i thesame as the intensity of the light incident on the comparisonphotoelectric cell. In the mentioned case, the device is so arrangedthat more light I falls on the photoelectric cell than on the film. Thisintensity can be roughly adjusted by a mechanical diaphragm CI. Theintensity of I is then greater than that of I and therefore Theresistors R for the feedback and the resistors R in the circuit of themeasuring photoelectric cell can be swiched over and they are in theratio 1:1, 10:1, 1000: 1. The output voltage U of the DC tube amplifierZ is reduced by the voltage divider R to a value U and this voltage isused by the calibrated voltage divider R for obtaining the compensationvoltage U Equation 4 becomes which is similar to Equation 4 except forthe first member term thereof. The first term equals zero if, in thefirst measuring range, that is R =R and U =U the slide contact being inthe upper position, the galvanometer H is adjusted to zero by thevoltage divider R without the inserted film (S=0), or with the insertedzero film (S=S Then U /U,,=k and log k.U,,/U =O. Therefore, the voltagedivider R serves for adjusting the fundamental zero position, eitherwithout the inserted film, or with the inserted zero film for measuringthe blackening only under the effect of radiation. The voltage divider Ris of the step-by-step type, controlled by a relay connected in parallelwith the output of the galvanometer H. Its contacts control the relayhunting system which automatically adjusts the necessary range, whichalready indicates the characteristic S, as well as the compensationvoltage U The position of the step-by-step voltage divider, in this caseone with two decades, indicates tenths and hundredths of S.

FIG. 3 shows the connection of the voltage divider R The points ofconnection 0, 1 and 2 correspond to equally marked points in the diagramin FIG. 2. Since the characteristic of the resistor is a linear one, theusual circuit arrangement cannot be used. For the first decade, theresistance of each section consists of two parallel resistors R0 and R0,R1, and R1, R2, and R2 and the like. All the resistors R0 to R9 have thesame value, and their lower ends are connected through the rest contactsof the relays P0 to P9 with the resistors R0 to R9. By alternate closingof the individual relays P0 to P9, the resistors R0 to R9 arealternately disconnected, and the resistors R0 to R9 are connectedinstead with the voltage divider of the second decade which consists ofresistors R00 to R09. The total resistance of this second voltagedivider equals one of the resistors R0 to R9 so that the resistance ofthe various sections of the first decade and the total resistance andcharacteristic do not change. The relay contacts P00 to P09 successivelychange over the slide contact of the potentiometer and divide thevarious sections of the first decade into an additional ten parts.

In accordance with Equation 4, the number of the range (1, 2, 3) formsthe characterisic of the blackening S, the position of the first decadethe first decimal place and the position of the second decade the seconddecimal place of S. The relays for changing over the ranges and theranges and the various decades of the voltage divider R also switch overlamps which indicate the blackening directly in numerals. The result ofthe measurement obtained by this method can be readily registeredautomatic ally on an electrical adding machine, or perforated into apunched card for statistical purposes.

The advantages of the digital photometer of the present invention areobvious. Low installation costs, absolute measuring method andpossibility of full automation are advantages. The measurement isindependent of the fatigue of the eyes of the operator. The results areexpressed by definite means, that is, by numerals. The device makes itpossible to register the results automatically by printing or by meansof punched cards of computing machines for purposes of registration andstatistics.

What I claim is:

1. A step-by-step nonlinear voltage divider comprising a plurality ofsections at least some of which have different resistance values fromeach other, each of said sections comprising a first resistor and asecond resistor connected in parallel, the first resistor of each ofsaid sections having the same resistance value;

an additional voltage divider having an overall resistance value whichis equal to the resistance value of said first resistor; and

switching means connected between the first and second resistors and theadditional voltage divider for disconnecting a selected one of the firstresistors of said sections from the corresponding one of said secondresistors and connecting said additional voltage divider.

2. A step-by-step nonlinear voltage divider as claimed in claim 1,wherein said switching means comprises relay changeover contacts.

(References 011 following page) References Cited UNITED STATES PATENTS 6OTHER REFERENCES Dean et al., A Digital Potentiometer, ElectronicChilton X Engineering, February 1956, vol. 28, No. 336, pp. 66 69. g gg323*80 5 JOHN F. COUCH, Primary Examiner.

Haanstra. G. GOLDBERG, Assistant Examiner.

Hock.

Brehm. US. Cl. X.R.

Morgan. 88-23; 323-94 Clapp. 10

Bergson.

